Clutches of this type, with spherical coupling elements biased by one or more springs into positive engagement with respective pairs of aligned part-spherical recesses of the two clutch members, are known for example from German printed Specification (Auslegeschrift) No. 1,101,059. In the event of an overload, i.e. when the resistance of the driven member exceeds the torque that can be safely transmitted by the driving member, these coupling elements are cammed out of their recesses against the biasing spring force and enter an endless track offset from those recesses. The width of the track may be greater than that of the recesses to minimize the resistance against relative rotation of the decoupled clutch members.
The re-engagement of the decoupled members of such a clutch requires a deactivation of the driving member and a realignment of their recesses with each other and with the spherical coupling bodies. Overload clutches are also known in which such realignment and re-engagement can be carried out automatically. Generally, however, these clutches are rather noisy when decoupled, owing to a ratchet effect which also subjects their parts to severe impacts and corresponding wear. In order to avoid this drawback, it has already been proposed (see East German Pat. No. 39, 404) to deactivate the clutch upon its disengagement by the opening of electrical contacts; this, of course, is possible only with electric drives.